docs: add AUDIT_PREP.md checklist for security audits

[blurbeast]

Closes #1916
resolves #1916
fixes #1916

Description

This PR introduces a comprehensive security audit preparation checklist (docs/AUDIT_PREP.md) targeted at contributors. It serves to standardize the preparation process, replacing tribal knowledge with a codified set of steps.

Changes Made

• Created docs/AUDIT_PREP.md with detailed instructions on what to hand over to the auditor, what to verify locally before handing over the codebase, and what to expect during the audit lifecycle.
• Included concrete examples for state transitions and commit hash formats instead of abstract definitions.
• Emphasized strict #![no_std] discipline, clearing out TODOs, and provided exact CLI commands for running WASM compilation, tests, and lints.
• Linked the new document from both README.md and docs/README.md to ensure it is highly discoverable.

Verification

• Verified cargo doc compilation instructions are correct.
• Checked markdown formatting and list hierarchy.
• All local CLI commands successfully build the WASM target and run contract tests as requested.